Lung infections of patients with cystic fibrosis often are caused by mucoid strains of Pseudomonas aeruginosa which produce an unusual extracellular polysaccharide. This material consists of poly beta-1,4-D-mannopyranosyluronic acid residues (ManU) with varying amounts of alpha-1,4 linked L-gulopyranosyluronic acid moieties (GulU) and resembles algal alginic acid. The biosynthesis of the polysaccharide has not yet been demonstrated; however, it may involve the following reactions. (1) nGDPMan plus 2nNAD ion yields nGDPMan u plus 2nNADH plus 2nH ion; enzyme, GDPMan Dehydrogenase. (2) nGDPMsnUA yields (ManU)n plus nGDP; enzyme, ManU polymerase. (3) (ManU)n yields (ManU)nm-(GulU)m); enzyme, D-mannuronosyl 5-epimerase. (4) (ManU)n-m)-(GulU)m) plus xAcSCoA yields (ManU-Acx)(n-m)-(GulU)m) plus xCoASH; enzyme, Acetylase. I propose to demonstrate each of these reactions in P. aeruginosa and to purify, at least partially, the enzymes responsible. One eventual aim will be to synthesize the polysaccharide in vitro. Understanding of the mechanism of the polysaccharide synthesis may provide information useful in combatting the harmful effects of the infection of cystic fibrosis patients by P. aeruginosa.